Bright plating of chromium



April 26, 1966 G. R. SCHAER 3,248,310

BRIGHT PLATING OF CHROMIUM Filed May 16, 1962 2 Sheets-Sheet 3 IIIIIII'IHHIIIIIlllllllllllllllll flllllllllllllllll IHIIIIH- |1|llllllllllllllllllllllllllllllllllll 11 |||m|||||||||m1 ||m|||| 45% fixfiep J L .5 77am v m zzrazmx;

United States Patent O 3,248,310 BRIGHT PLATING F CHROMIUM Glenn R. Schaer, Columbus, Ohio, assignor, by mesne assignments, to General Development Corporation, Miami, Fla, a corporation of Delaware Filed May 16, 1962, Ser. No. 195,106 9 Claims. (Cl. 204-51) This invention relates to chromium plating and more particularly the invention relates to a chromium plating bath composition and the method of employing such bath composition in depositing a bright chromium plate over bright nickel and other bright metal surfaces.

Chromium is well known as a final electroplate finish on other metals, especially because it retains a high luster and does not tarnish. Chromium itself has the capacity to resist corrosion and is therefore desirable in providing a corrosion resistant surface.

In the decorative-protective finishing of metals such as steel, brass, copper, zinc die castings for example, to prevent rusting, tarnishing or other forms of corrosion, it is well known to apply multi-layer electroplates. For example, copper and/ or nickel and chromium on steel, nickel and chromium on brass and copper, and copper, nickel and chromium on zinc die castings.

The copper and/or the nickel plates may he applied in dull mat appearance and be buffed to mirror-like luster before chromium plating. The more usual practice is to bright finish the basis metal object and then apply bright electroplates. Practical methods are in wide commercial use and are well known in the art for electrodepositing bright copper and bright nickel with mirror-like luster needing no buffing or coloring before chromium plating. The appearance after chromium plating is the brilliant mirror-like quality that the art has become accustomed to designate as a bright chromium finish.

Objectively, the bright chrome finish is expected to retain its attractiveness as long as the life expectancy of the object so finished. Whereas only five years ago, the bright chrome finish on zinc die casting and on steel parts showed unsightly deterioration in a short time as three months in outdoor service on automobiles, advances during the last five years have shown how to achieve two years and longer duration of the bright plate without objectionable deterioration of the bright chrome appearance. This progress was made possible by the use of duplex nickel plate in place of only bright nickel plate and by recognizing how to make fullest use of chromium plate.

The present invention relates to a novel way to attain improved chromium plate. The improved chromium plating technique is especially significant in the decorative plating on bright nickel of parts of complicated shape.

Chromium plating baths of the known art have notably poor covering power and throwing power. Also, most of the known processes deposit chromium that is porous when the thickness is less than about 0.02 mil (0.00002 inch) thick and is macro cracked when the thickness exceeds about 0.03 mil. Furthermore, the prior art chromiurn plating baths have higher current efiiciency at higher current densities. This characteristic accounts for the abnormally poor throwing power and for a limiting current density below which no chromium metal deposits. At such below minimum current densities a rainbow stain may deposit. This limiting minimum current density is much higher than the limiting minimum current density at which bright nickel will deposit in mirror-like form.

As a basis for understanding the present plight of the chromium plater, as well as an appreciation of the soluduplex nickel on protruding surface.

'minimum plate regions.

tion which the present invention provides for that plight, the following discussion should be considered:

First, consider for example, the plating of bright nickel on a shaped part having deep recesses as well as relief or protruding areas. When the average plating current density is 50 amps per square foot, the current density in the recessed areas is on the order of 10 amps per square foot whereas in the relief areas the maximum current density will be approximately amps per square foot. In this example, a primary current density between the recessed and relief areas is between 1 and 7.5. The current efficiency of the nickel deposition is the same at 10 as at 75 amps per square foot, that is, about 98%. Therefore, the plate thicknesses will be in the ratio of 1 to 7.5 at the low and high current density areas respectively.

For acceptable corrosion protection and retention of appearance, minimum plate thicknesses are specified. Such thicknesses are well established in the art and for duplex nickel plate the minimum plate thicknesses should be 0.8 mil. Therefore, electroplating under conditions to give 0.8 mil duplex nickel in the recess of the above mentioned shaped part applies 7.5 times as much, or 5.6 mil This is the best that can be achieved because it is decided by the primary current distribution. The electroplater has means at his disposal such as shielding and special racking techniques to shift the primary current ratio to a more favorable value of 1 to 3 to 1 to 4. Then the thickness distribution of duplex nickel plate is 0.8 mil and 2.4 to 3.2 mil.

The above discussion on nickel plating is the foundation for seeing the problem facing platers with only the prior art chromium plating baths. The best of them does not deposit chromium metal at current density below 20 amps per square foot. At current densities near this current density (but higher, the current efficiency of chromium deposition is about five percent. When the average chromium plating current is amps per square foot and the current density at the protruding or relief surface is about 225 amps per square foot, the current efiiciency of bright chromium metal deposition at the higher current density will be about 14 percent. Thus, the higher current efliciency exaggerates the thickness variation for chromium plate at low current density regions relative to chromium plate at the high current density regions.

-N ow, consider the previously described part which has bright duplex nickel plate in uniform brilliance all over and which is to be bright chromium plated. There will be no chromium metal color at the low current density region which, therefore, will have either the color of the nickel or a rainbow stain. Near this low current density region the chromium plate will be 0.002 to 0.005 mil thick and at the high current density region will be 0.06 mil; a thickness ratio of to This thickness ratio is arrived at by calculation for five minute chromium plating using any table in one of the handbooks that give data on deposition rates for a range of current densities for chromium plating.

The plight of the plater is clearly revealed. He can provide a uniform bright duplex nickel appearance and plate distribution satisfactory for corrosion protection but he cannot achieve uniform chromium plate for equal corrosion protection all over. The chromium is macro cracked at the thicker plate regions and porous at the Both conditions have been recently shown in the plating literature to significantly diminish the protective durability of the electroplate. The plater can resort to techniques of bipolar electrodes, current thieves, current shields high current densities and by juggling the temperature and sulfate ratio which are costly and makeshift at best, to get better chromium distribution. Most of his gain will be at the high current density regions, with little or none at the low current density regions.

Clearly, the plating industry would have a significant advance if bright chromium could be electrodeposited with the same throwing power as bright duplex nickel and of bright nickel plate. The novel chromium plating bath described herein closely approaches achievement of this desirable and practical accomplishment.

An objective of the invention has been to improve the covering power and throwing power in a chromium plating process.

Another objective of the invention has been to tend to decrease the eflieiency of the chromium plating at the edges of a part While increasing the elliciency of the deposition in the part recesses.

These objectives will become more readily apparent from the following detailed description taken with the accompanying drawings in which:

FIG. 1 illustrates the preferred and operable ranges of .trichloroacetic acid and sulfate ion;

FIG. 2 illustrates the preferred and operable ranges of monochloroacetic acid and sulfate ion; and FIG. 3 shows a variation in plate thickness versus current density from diflferent chromium plating baths.

Chromium plating has since the 1920s been practiced by employing a solution containing chromic acid and sulfate ion, the ratio of chromic acid to sulfate ion by weight being maintained approximately in the range of 80/1 to 120/ 1.

The literature is filled with attempts to improve the efiicacy of the chromium plating bath, some of these efforts taking the form of the introduction of additives to the standard chromium plating bath described above and others being the substitution of different compounds for the sulfuric acid by which the sulfate ion has been introduced. In at least one instance trichloroacetic acid was combined with chromic acid, the sulfate ion having been eliminated. While at least some of these attempts have unquestionably improved the chromium plating process for certain purposes, the fact remains that, prior to the present invention, the throwing power of the chromium plating bath has not approached that of nickel, the covering power of chromium plating has not approached that of bright nickel and it has not been possible to plate lustrous thick, crack-free, layers of chromium over bright nickel while also providing throwing power and covering power comparable to that of nickel plate. quence of these infirmities, it has been possible to deposit a satisfactory corrosion resistant and decorative chromium plate only on parts of very simple shape.

It has been found that a marked and quite unexpected improvement in the overall characteristics of a bright chromium deposit can be attained by employing, in the chromium plating bath, the combination of chromic acid, sulfate ion and a halogenated acetic acid, the chromic acid to sulfate ratio being in the range of 200/1 to 300/1.

One of the surprising effects is exemplified by FIG. 3. FIG. 3 is a plot of plate thicknesses versus densities for trichloroacetic (TCA) catalyst, S catalyst and combined TCA and S0 catalysts. The thicknesses were measured on Hull cell panels utilizing the process described in Patent No. 2,149,344. From FIG. 3 it can be observed that increases in sulfate ion alone or trichloroacetic acid alone to a chromium plating bath tend to increase the slope of the current efiiciency curve. The surprising effect is indicated by the curve of the combined TCA and sulfate ion, that is, the curve of the bath of the present invention. By combining the two additives, the etficiency of the bath at very low current densities (less than 30 amps. per sq. foot) has been improved, but the As a come-- efiiciency of the bath at high current densities has been markedly decreased.

Ideally, this is the direction in which the curve or plot of efficiency should move in order to improve the efficacy of the bath. In order to plate shaped parts, the best results are obtained when the current effieiency at high current densities is less than the efiiciency of the bath at low current densities. With such a bath, the protruding or relief areas do not build up chromium at such a great rate whereas it is possible to deposit greater thicknesses of chromium in the recessed areas.

The covering power or minimum current density at which chromium can be electr'o-deposited when both TCA and sulfate are used as catalyst is about 4 amp/sq. ft.; whereas the minimum current density at which chromium can be deposited is about 20 amp./sq. ft. when only 3 g./l. sulfate is the catalyst. The significance of these numbers is that the total current needed to cause plating of chromium in a recess of a shaped part will be about 7 /2 times as much for the sulfate-only-catalyst bath as for the mixed sulfate-TCA-catalyst bath.

The preferred range of constituents for the plating bath is as follows:

In the plating bath described above, the presence of sulfate ion is absolutely essential. While it is possible to obtain a chromium deposit without the sulfuric acid or sulfate ion, it is not possible to obtain anything approaching a satisfactory corrosion resistant bright plate over bright nickel when sulfate ion is absent, the only catalyst being the halogenated acetic acid.

Considerable criticality attaches to the sulfate ratio, that is the ratio of CrO to H If the ratio is substantially in excess of 300/1, as for example 400/1 or 600/1, a number of undesirable effects are noted. For example, the covering power suffers to the extent that it is quite difiicult to plate in deep recesses. Also, it is not possible to obtain bright plates at high current densities which will occur at the projecting portions of a shaped work piece. The chromium plate will be dull or burned.

If the sulfate ratio is too low, again the covering power suffers and the resultant plates at the areas of high current densities become cracked. Substantial deviations from the prescribed ranges of halogenated acids results in both throwing power and covering becoming poorer.

The relationship of the sulfuric acid content and the trichloroacetic acid content in a bath containing 300 g./l. of chromic acid is delineated approximately in the graph of FIG. 1. The area defined by the lines A, B, C, D, E, and F is the operable range. Within that area, the preferred range is defined by the lines G, H, I and I. Quite satisfactory plates can be deposited on articles which normally would be extremely difficult to plate with chromium if the sulfuric acid and trichloroacetic acid catalysts are maintained in the proper relationship and in the preferred ranges as defined by the area G, H, I, J. Operation is also possible outside of the preferred range as long as the operation is maintained within the operable range as defined by the area A, B, C, D, E, F. However, it must be appreciated that with parts having deep recesses or unusually diflicult to plate shapes as for example sharp edges or protrusions, the resulting plates may not be completely satisfactory when deposited in a bath when the constituents are outside of the preferred range.

In other words, the operable range has the utility for parts which are not toodifficult to plate and, indeed, the resulting plates will have properties which are preferable to plates obtainable by the best known prior art plating baths.

EXAMPLE I A plating bath was made up as follows:

G./l. Chromic acid 300 Sulfuric acid 1.2 Trichloroacetic acid 15 Trivalent chromium 1.5

The bath was maintained at a temperature of between 125 and 127 F. and plating current was applied between the anode and a workpiece at approximately 288 amps per square foot. The workpiece had previously been plated with copper and bright nickel prior to chromium plating. Current was applied for five minutes and a bright lustrous chromium was deposited on the surface of the workpiece, the thickness of the chromium being as high as 0.16 mil without any cracks appearing and with a satisfactory bright lustrous appearance.

EXAMPLE II Other halogenated acetic acid catalysts may be used instead of trichloroacetic acid. Bright nickel plated panels were plated with chromium in a bath of the following composition:

G./l. Chromic acid 300 Sulfuric acid 1.2 Monochloroacetic acid (MCA) 45 The operating conditions were the same as in Example I.

The resultant plate deposited on the panel was a fully acceptable quality having the characteristics of being EXAMPLE III Chromium was deposited over bright nickel in a bath of the following composition:

G./l. Chromic acid 300 Tn'bromoacetic acid 20 Sulfuric acid 1.25

EXAMPLE 1V Monoiodoacetic acid was employed as a catalyst in a bath of the following composition:

Chromic acid 300 Monoiodoacetic acid 20 Sulfuric acid 1.25

The panels were plated at a bath temperature of 92 F. for five minutes at a current density of 90 a.s.f. (The current density here is lower than that normally employed. However, the instability of MIA required a substantial deviation from the usual conditions.)

While satisfactorily plating results were obtained it was found that the bath employing MIA was not as stable as the TCA, MCA and TBA baths.

A summary of further examples appears in the following Table 1. In those examples the chromium was plated on panels which previously had a bright nickel deposit. The plating bath, in addition to the particular additive set forth, contained 300 g./l. CrO and 1.2 g./l. H 80 and 1.0 g./l. Cr.

Table 1.-Hal0genaled acetic acid catalysts Concentra- Plating Average cur- Temp., Compound tion, g./l. time, rent density, F. Plate appearance min. amp/sq. ft.

Moriigfluoroaoetie acid (sodium 4 5 288 120 Bright plate.

sa Difluoroacetie acid 4 5 288 120 D0. Tritluoroacetic acid 4 5 288 125 D0. Monochloroacetic acid 5 288 125 Do. Dichloroacetic acid 40 5 288 125 Do. Trichloroacetic acid 15 5 288 125 Do. Monobromoacetic acid 4 5 288 120 Do. Drbromoacetic acid..- 4 5 288 120 Do. Tribromoacetic acid 4 5 288 120 Do. Monoiodoacetic acid. 4' 3 90 92 D0. Diiodoacetic acid 4 3 90 91 D0. Triiodoacetic acid -1 2 3 90 90 Bright Plate (Slightly milky on edges). None 288 125 Brown spots scattered on bright plate. Mlxture of mono, di, trichloro- 5 288 125 Full bright plate.

acetic acids.

1 l2 g./l. each.

crack-free, bright and lustrous. Additional panels were plated under varying conditions and it was found that the throwing power, covering power and ability to deposit a crack-free chromium were much better than conventional plating baths currently in use and were substantially equivalent to the trichloroacetic acid-sulfuric acid bath. In addition, the concentration ranges for the monochloroacetic acid are higher than for trichloroacetic acid. The trichloroacetic acid has superior plating characteristics at low current densities and is therefore preferred. The relationship and ranges of the sulfuric acid to the monochloroacetic acid in a bath containing 300 g./l. of

chromic acid is illustrated in FIG. 2. The operable range In the practice of the instant invention in electrolytic systems wherein lead anodes are employed, there is a tendency for the plating solution to corrode the anodes and to form a contaminating sludge in the plating tank. This problem can be obviated by utilizing anodes other than lead which do not react with any of the solution constituents. Alternatively, a small amount of cobaltous ion can be added to the solution, preferably in the form of cobalt sulfate, the utilization of cobaltous ion being a well known expedient to inhabit the corrosion of l ad anodes in other electrolytic systems.

In connection with the practice of the invention, it may be desirable to package for shipment and/or sale differing types of mixtures of the plating solution constituents. These constituents have a synergistic action when dissolved in water and when subjected to a voltage applied between an anode and a cathode constituted by an article to be chromium plated.

In preparing and employing a commercial package of the constituents, it should be understood that the various salts are not normally chemically pure. For example, the chromic acid will normally contain a small amount of a sulfate salt. Further, the commercial grade of trichloroacetic acid will contain approximately 98% trichloroacetic acid and 2% dichloroacetic acid. It should therefore be understood that the examples given hereafter are approximations and that a bath prepared from the following examples will normally be analyzed and additional salts added to bring the concentrations within the preferred ranges prescribed above.

It should also be understood, as is well known in the art, that the sulfate ion can be added as sulfuric acid or as a salt such as chromium sulfate which, when dissolved in the solution, will act identically as if it were introduced as sulfuric acid.

The following are examples of compositions adaptable for commercial packaging and the manner in which they are added to water to provide a chromium plating bath in which the concentrations fall within the desired ranges.

EXAMPLES V, VI AND VII Three baths of IO-liter volume were made up by dissolving salts in the following proportion:

Exam 1e rams Chemicals used p (g V VI VII Chromic acid Trichloroacetic acid Cobalt sulfate Chromium sulfate Total These chemicals can be combined as mixtures of dry salts in the proportions shown. Each plating bath is prepared by dissolving the mixed salts in the right amount of water to give liters of solution of the following compositions:

Exam 1e rams Composition of baths in grams per liter p g VI VII Chromic Acid (CrO 3 T A Sulfate from chromic acid Sulfate from cobalt salt Sulfate from chromium salt Sulfate total 9. r 99.9 m Mo was Chromium trivalent Thus, three examples show essentially using 317 grams of the mixed salts per liter of plating bath. No additional sulfuric acid is needed.

EXAMPLE VIII critical than the sulfuric acid content.

8 EXAMPLE 1x A satisfactory plating bath can be made by dissolving 316.5 grams of a salt mixture containing 300 grams of chromic acid and 16.5 grams of the chromium salt of trichloroacetic acid [Cr(CCl -COO per liter of bath. After dissolution of the mixture, sulfuric acid should be added to give a CrO :SO.,= ratio of 250: 1.

EXAMPLE X A satisfactory plating bath can be made by dissolving from to 450 g./l. of a salt mixture containing from 2.4 to 12.9 percent trichloroacetic acid, 1 percent water for a binder, and the balance chromic acid; then when the salts are dissolved, adding sulfuric acid to the solution .to adjust the chromic acid-sulfuric acid ratio to 250:1.

EXAMPLE XI A satisfactory plating bath can be made by dissolving 100 to 450 grams of a salt mixture containing 300 parts chromic acid and 7.5 to 45 parts trichloroacetic acid and 1.7 to 2.5 parts of chromic sulfate hydrate EXAMPLE XII A satisfactory plating bath can be made by dissolving 100 to 450 grams of a salt mixture containing 300 parts chromic acid, 10 to 20 parts trichloroacetic or monochloroacetic or tribromoacetic acid. After dissolution of the mixture, sulfuric acid should be added to give a CrO :SO ratio of 250:1.

As can be seen from the examples, all of the halogenated acetic acids, when added to a bath containing chromic acid and sulfuric acid with a sulfate ratio between 200/1 and 400/1 will provide a satisfactory bright plate. Additionally, andmost important, it is the fact that by combining the condition of high sulfate ratio with the halogenated acetic acid in the chromium plating bath, the coveringpower is greatly improved and the throwing power is noticeably improved to approach or equal the covering power and throwing power of nickel. Thus, for example in the plating of shaped parts such as bezels for instruments for automobile dashboards and the like where it is possible to cover deep recesses by bright nickel, it is also possible by the present novel process to cover the nickel with a satisfactory plate of bright chromium. Heretofore this has not been possible.

As indicated above, it should be understood that deviations from the specific concentrations and operating conditions of the chromium plating bath are permitted and fall within the scope of the invention. The effect of certain deviations has been noted and will be discussed below.

SULFURIC ACID CONTENT,

The sulfuric acid content is an important variable and the most sensitive in that small changes from the preferred 1.2 g./l. of the absolute amount of sulfate (as small as 0.2 g./l.) can change covering and throwing power. A change of 0.2 g./l. is a 16% percent change in the sulfate, which is a significant change in the composition. With sulfate contents higher than 1.2 g./l. covering power and throwing power are less desirable and the thickness limit of crack-free plate is significantly lessened. With lower sulfate contents, that is, about 0.75 to 1.0 g./1., the

4 covering and throwing power are less desirable, but the crack-free limit is raised. With sulfate contents much below 0.75 g./1. the chromium plated at high current densities is not bright.

HALOGENATED ACETIC ACID CONTENT The content of the halogenated acetic acid is much less For example, bright plates can be made with TCA contents from 7.5 to 45 g./l. However, the best combination of covering and throwing power are obtained at 15 g./l. The

crack-free limit is highest at the 7.5 g./l. content and lowest at the 45 g./l. content. The halogenated acetic acid is consumed slowly during plating and a small amount of the halogen ion is liberated. This decomposition might be the reason for the unexpected and advantageous results of this mixture of chemicals. In this respect, the halogenated acetic acids are not true catalysts because a true catalyst is not consumed.

CHROMIC ACID Changing the chromic acid content has a larger effeet on covering power and the thickness limit of crackfree plate than on throwing power. The optimum covering power seems to be derived from a 300 g./l. chromic acid bath. Concentrations higher than 300 g./l. will increase the crack-free limit but decrease the covering power. Concentrations lower than 300 g./l. (such as 150 g./l.) will reduce both covering power and crackfree limit. In these discussions it is assumed that the chromic acid-sulfuric acid ratio remains constant.

TEMPERATURE The major effect of changing temperature is on covering power and thickness limit of crack-free plate. Lower temperatures (less than 125 P.) will improve covering but greatly decrease the crack-free limit. The optimum temperature for all around performance is considered to be 125 F. If covering power is the most important factor, such as is needed for plating for color on complex shaped parts for non-corrosive environmental use, then lower temperatures such as 110 F. would be desirable.

I claim:

1. The process of chromium plating an article comprising passing a current between an anode and an article forming a cathode which are immersed in an aqueous chromium plating bath consisting essentially of the following:

G./l. Chromic acid 285-315 Sulfuric acid (as H 80 1.0-1.5 Hal ogenated acetic acid 2-45 2. The process of chromium plating an article comprising passing a current between an anode and an article forming a cathode which are immersed in an aqueous chromium bath consisting essentially of the following:

Chromic acid G./l 285-315 Sulfuric acid (as H 50 G./l 1.0-1.5 Gro /sulfate ratio 200/1 to 300/ 1 Halogenated acetic acid G./l 2-45 3. The process of chromium plating an article comprising passing a current-between an anode and an article forming a cathode which are immersed in an aqueous chromium plating bath consisting essentially of the following:

Chromic acid G./l 285-315 Sulfuric acid (as H 50 G./l 1.0-1.5 CrO /sulfate ratio 200/ 1 to 300/1 Trichloroacetic acid approximately G./l

4. The process of chromium plating an article comprising passing a current between an anode and an article forming a cathode which are immersed in an aqueous chromium plating bath consisting essentially of the following:

Chromic acid G./l 285-315 Sulfuric acid (as H 80 G./l 1.0-1.5 CrO sulfate ratio 200/1 to 300/1 and between 4 and 45 g./'l. of a substance selected from the group consisting of monfluoroacetic acid, difluo'roacetic acid, trifluoroacetic acid, monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, dibromoacetic acid, tribromoacetic acid, and mixtures of any of said halogen substituted acetic acids.

5. The process of chromium plating an article comprising passing a current between an anode and an article forming a cathode which are immersed in an aqueous chromium plating bath consisting essentially of the following:

Chromic acid G./l 285-315 Sulfuric acid (as H SO.,) G./l 1.0-1.5 CrO sulfate ratio 200/1 to 300/1 Trichloroacetic acid G./l 10-20 said bath being maintained at a temperature of approximately 125 F. and the average current density on said cathode being approximately 288 a.s.f.

6. A composition for use in an aqueous chromium plating solution said composition consisting essentially of: 300 parts by weight CrO 4 to 45 parts by weight halogenated acetic acid, and

a sulfate salt wherein the sulfate contributes approximately 1.2 parts by weight of the total weight of said composition.

7. A composition for use in an aqueous chromium plating solution said composition consisting essentially of:

300 parts by weight CrO 4 to 45 parts by weight halogenated acetic acid, and

a sulfate salt wherein the sulfate contributes approximately 1.2 parts by weight of the total weight of said composition,

said halogenated acetic acid being selected from the group consisting of mono-, diand trifluoroacetic acid, monodiand trichloroacetic acid, mono-, diand tribromoacetic acicl1 and mixtures of any of said halogen substituted acetic aci s.

8. The process of chromium plating an article comprising passing a current between an anode and an article forming a cathode which are immersed in an aqueous chromium plating bath consisting essentially of chromic acid, sulfuric acid, and halogenated acetic acid in which chromic acid/sulfuric acid ratio is 100/1 to 600/1 and halogenated acetic acid is 2 to 45 g./l.

9. A chromium plating bath consisting essentially of 285-315 g./l. chromic acid, 1.0 to 1.5 g./l. sulfuric acid,

10 to 20 g./l. trichloroacetic acid, and up to about 2 g./l. trivalent chromium.

References Cited by the Examiner UNITED STATES PATENTS JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, Examiner. 

1. THE PROCESS OF CHROMIUM PLATING AN ARTICLE COMPRISING PASSING A CURRENT BETWEEN AN ANODE AND AN ARTICLE FORMING A CATHODE WHICH ARE IMMERSED IN AN AQUEOUS CHROMIUM PLATING BATH CONSISTING ESSENTIALLY OF THE FOLLOWING: 